1. Field of the Invention
The present invention relates to a heterocyclic compound. In particular, the present invention relates to a heterocyclic compound that can be used for a light-emitting element utilizing organic electroluminescence (EL).
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements utilizing EL. In the basic structure of such a light-emitting element, a layer containing a light-emitting substance is interposed between a pair of electrodes. By application of a voltage to this element, light emission from the light-emitting substance can be obtained.
Since such light-emitting elements are of a self-light-emitting type, it is considered that they have advantages over liquid crystal displays that the visibility of pixels is high, backlights are not required, and so on, and therefore the light-emitting elements are suitable as flat panel display elements. The light-emitting elements also have a great advantage that they can be manufactured as thin and lightweight elements. Further, very high-speed response is also one of the features of such elements.
Furthermore, since such light-emitting elements can be formed in a film form, they make it possible to provide planar light emission. Therefore, large-area elements utilizing planar light emission can be easily formed. This feature is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, light-emitting elements also have great potential as planar light sources applicable to lighting devices and the like.
Such light-emitting elements utilizing electroluminescence can be broadly classified according to whether the light-emitting substance is an organic compound or an inorganic compound. In the case of an organic EL element in which a layer containing an organic compound used as the light-emitting substance is provided between a pair of electrodes, application of a voltage to the light-emitting element causes electron injection from a cathode and hole injection from an anode into the layer containing the organic compound having a light-emitting property and thus current flows. The injected electrons and holes then lead the organic compound to its excited state, so that light emission is obtained from the excited organic compound.
The excited state formed by an organic compound can be a singlet excited state or a triplet excited state. Light emission from the singlet excited state (S*) is called fluorescence, and emission from the triplet excited state (T*) is called phosphorescence. In addition, the statistical generation ratio thereof in a light-emitting element is considered as follows: S*:T*=1:3.
At room temperature, an observation on a compound that can convert energy of a singlet excited state into light emission (hereinafter, referred to as a fluorescent compound) usually shows only light emission from the singlet excited state (fluorescence) without light emission from the triplet excited state (phosphorescence). Therefore the internal quantum efficiency (the ratio of generated photons to injected carriers) of a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% based on a S*-to-T* ratio of 1:3.
In contrast, an observation on a compound that can convert energy of a triplet excited state into light emission (hereinafter, called a phosphorescent compound) shows light emission from the triplet excited state (phosphorescence). Further, since intersystem crossing (i.e., transition from a singlet excited state to a triplet excited state) easily occurs in a phosphorescent compound, the internal quantum efficiency can be theoretically increased to 100%. That is, higher emission efficiency can be obtained than using a fluorescent compound. For this reason, light-emitting elements using a phosphorescent compound have been under active development recently in order that highly efficient light-emitting elements can be realized.
When formed using the above-described phosphorescent compound, a light-emitting layer of a light-emitting element is often formed such that the phosphorescent compound is dispersed in a matrix containing another compound in order to suppress concentration quenching or quenching due to triplet-triplet annihilation in the phosphorescent compound. Here, the compound as the matrix is called a host material, and the compound dispersed in the matrix, such as a phosphorescent compound, is called a guest material.
In the case where a phosphorescent compound is a guest material, a host material needs to have higher triplet excitation energy (energy difference between a ground state and a triplet excited state) than the phosphorescent compound.
Furthermore, since singlet excitation energy (energy difference between a ground state and a singlet excited state) is higher than triplet excitation energy, a substance that has high triplet excitation energy also has high singlet excitation energy. Therefore the above substance that has high triplet excitation energy is also effective in a light-emitting element using a fluorescent compound as a light-emitting substance.
Studies have been conducted on compounds having dibenzo[f,h]quinoxaline rings, which are examples of the host material used when a phosphorescent compound is a guest material (e.g., see Patent Documents 1 and 2).